The only way I can think to stop that behavior is to turn on some sort of light, they don't like sleeping in light, they prefer to sleep in the dark, which the nest box with be. Eventually they will get used to the nest box and sleep in that. If they still sleep in the food, try putting some dry food like oatmeal in the dish that you have for them to sleep in, since they seem to enjoy sleeping in food, maybe they will sleep in this. An added bonus to oatmeal is it makes them smell better, my gliders love to eat it too.